Optimal use of request access TDMA slots for automatic level control

ABSTRACT

Systems and methods for controlling subscriber unit power level using power measurements made on access request transmissions. Since a subscriber unit access request typically precedes a subscriber unit data transmission by a relatively short period of time, the data transmission power level will then be based on a recent power measurement. The access request may also trigger a sequence of power control steps including the transmission of special upstream power measurement transmissions which can be used as the basis for even more accurate power control. These power control features are particularly useful in fading environments such as found in a wireless system.

STATEMENT OF RELATED APPLICATIONS STATEMENT OF RELATED APPLICATIONS

The present application is related to the subject matter of thefollowing co-filed, coassigned applications.

U.S. patent app. Ser. No. 09/348,646, WIRELESS POWER CONTROL INCONJUNCTION WITH A WIRELINE MAC PROTOCOL;

U.S. patent app. Ser. No. 09/348,647, COMMUNICATION OF PHYSICAL LAYERCONTROL PARAMETERS

U.S. patent app. Ser. No. 09/348,644, REALTIME POWER CONTROL IN OFDMSYSTEMS;

U.S. patent app. Ser. No. 09/348,719, POWER REGULATION USING MULTI-LOOPCONTROL;

U.S. patent app. Ser. No. 09/348,727, POLLING FOR TRANSMISSION POWERCONTROL;

U.S. patent app. Ser. No. 09/348,718, EFFICIENT REQUEST ACCESS FOR OFDMSYSTEMS.

The present application is also related to the subject matter of U.S.app. Ser. No. 09/019,938, filed Feb. 6, 1998, titled MEDIUM ACCESSCONTROL PROTOCOL FOR OFDM WIRELESS NETWORKS.

All of the related applications are incorporated by reference herein forall purposes.

BACKGROUND OF THE INVENTION

The present invention is related to digital communication systems andmore particularly to systems and methods for controlling output power ofsubscriber units in a point to multipoint communication system.

A point to multipoint wireless communication system represents apotentially effective solution to the problem of providing broadbandnetwork connectivity to a large number of geographically distributedpoints. Unlike optical fiber, DSL, and cable modems, there is no need toeither construct a new wired infrastructure or substantially modify awired infrastructure that has been constructed for a different purpose.

In order to conserve scarce spectrum, the data communication devices ofa point to multipoint wireless communication system may share access toa common frequency. In a typical scenario one or more frequency channelsare allocated to downstream broadcast communication from a centralaccess point to a plurality of subscriber units and one or more separatefrequency channels are allocated to upstream communication from thesubscriber units to the central access point. For upstream communicationthere is a medium access control (MAC) protocol that determines whichsubscriber unit is permitted to transmit at which time so as not tointerfere with transmissions from other subscriber units.

For a given upstream frequency, the time domain is divided into frameswhich are typically of equal duration. Each frame represents anindividually allocable unit in the time domain. One subscriber unittransmits in each frame. Reservations for transmission in a particularframe are made by the central access point and distributed in broadcastdownstream transmissions. Such a scheme is referred to as a time domainmultiple access scheme (TDMA).

In such a point to multipoint wireless communication system, it isgenerally preferable to centrally control the transmission power of eachsubscriber unit. Each subscriber unit should transmit at a powersufficient to ensure accurate reception its transmission yet not so highso as to overload the front end of the central access points receiver orcause interference to unintended receivers. Power control involvesmonitoring received power for each subscriber unit at the central accesspoint and sending power adjustment information downstream to maintainpower at the desired level.

Cable modem systems also involve access to a shared medium and alsorequire subscriber unit power control. It would be desirable to simplyadopt a MAC protocol already developed for cable applications to thewireless context. One such protocol that has been developed is referredto as MCNS protocol. The MCNS protocol is described in theData-over-Cable Service Interface Specifications, Radio FrequencyInterface Specification, SP-RFI-I04-980724, (Cable TelevisionLaboratories, 1997), the contents of which are herein incorporated byreference.

A cable MAC layer like MCNS is already implemented in low cost chipsets. The operational characteristics of MCNS are well known.Furthermore, it is desirable to maintain parts commonality betweenwireless modems and cable modems to the extent possible.

The MCNS protocol provides for controlling the power of subscriberunits. In one implementation, the power control function is combinedwith monitoring of the round trip propagation delay between the centralaccess point and individual subscriber units. Periodically, the centralaccess point sends a ranging request message to a particular subscriberunit. In response to the ranging request message, the subscriber unitsends a ranging response to the central access point. Based on thistransmission, the central access point establishes a round trippropagation delay and sends this value to the subscriber unit. Thecentral access point measures the power level of the ranging responsemessage. Based on the power measurement, the central access point sendsthe subscriber unit power adjustment information to help the subscriberunit set its power so that it will be received at a desired level.

This combined ranging and power control operation is, however,relatively infrequent, occurring approximately every two seconds in atypical implementation. This MAC layer power control operation cannoteasily be made more frequent because of the limited processing powerprovided by equipment implementing the MCNS protocol. Also, eachsubscriber unit's ranging response requires a MAC frame, causingfrequent updates to reduce system efficiency.

In a wireless system, the frequency of power control feasible with theuse of MCNS power control features is insufficient due to the inherentlymore rapid variation in wireless channel response over time. What isneeded are systems and methods for providing more rapid update ofsubscriber unit power level while still interoperating with wireline MACprotocols.

SUMMARY OF THE INVENTION

Systems and methods for controlling subscriber unit power level usingpower measurements made on access request transmissions are provided byvirtue of the present invention. Since a subscriber unit access requesttypically precedes a subscriber unit data transmission by a relativelyshort period of time, the data transmission power level will then bebased on a recent power measurement. The access request may also triggera sequence of power control steps including the transmission of specialupstream power measurement transmissions which can be used as the basisfor even more accurate power control. The power control features of thepresent invention are particularly useful in fading environments such asfound in a wireless system.

In accordance with a first aspect of the present invention, a method foroperating a central access point in a point to multipoint communicationsystem is provided. The method includes receiving an access requesttransmission from a subscriber unit and performing a power controlsequence to control output power of the subscriber unit in response tothe access request transmission.

In accordance with a second aspect of the present invention, a methodfor operating a subscriber unit in a point to multipoint communicationsystem is provided. The method includes sending an access requesttransmission to a central access point, and receiving a first poweradjustment information indicator from the central access point. Thefirst power adjustment information indicator has been generated as partof a procedure performed responsive to the access request transmission.The method further includes receiving a scheduling transmission from thecentral access point. The scheduling transmission has been generated inresponse to the access request transmission and specifies a time of afuture transmission by the subscriber unit. The method further includessending the future transmission to the central access point at a powerlevel determined in response to the power adjustment command.

Further understanding the nature and advantages of the invention hereinmay be realized by reference to the remaining portions of thespecification and the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a point to multipoint wireless communication systemaccording to one embodiment of the present invention.

FIG. 2 depicts interaction between a central access point and arepresentative subscriber unit of a point to multipoint communicationsystem according to one embodiment of the present invention.

FIG. 3 depicts a sequence of power measurements in the time domainaccording to one embodiment of the present invention.

FIG. 4 is a flowchart describing steps of controlling subscriber unitoutput power according to one embodiment of the present invention.

FIG. 5 depicts a sequence of power measurements in the time domain in analternative embodiment of the present invention.

FIG. 6 is a flowchart describing steps of controlling subscriber unitoutput power according to an alternative embodiment of the presentinvention.

FIGS. 7A-7B depict alternative structures of an access request OFDMburst according to one embodiment of the present invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS

FIG. 1 depicts a point to multipoint wireless communication network 100suitable for implementing one embodiment of the present invention.Network 100 includes a central access point or head end 102 and multiplesubscriber units 104. All communication is typically either to or fromcentral access point 102. Communication from central access point 102 toone or more subscriber units 104 is herein referred to as downstreamcommunication. Communication from any one of subscriber units 104 tocentral access point 102 is herein referred to as upstreamcommunication. In one embodiment, different frequencies are allocated toupstream and downstream communication. In alternate embodiments,subscriber units 104 may communicate with one another directly.

Each of one or more upstream frequencies is common to multiplesubscriber units. To prevent collisions between subscriber units whenaccessing the shared medium, a medium access control (MAC) protocol isprovided. According to one embodiment of the present invention, a MACprotocol intended for data transmission over cable systems may be usedto coordinate upstream communications in wireless network 100. Anexemplary MAC protocol of this type is the so-called MCNS protocoldescribed in the Data-over-Cable Service Interface Specifications, RadioFrequency Interface Specification, SP-RFI-I04-980724, (Cable TelevisionLaboratories, 1997).

MCNS employs a time domain multiple access (TDMA) scheme to allocateaccess to the shared upstream frequency among multiple subscriber units104. The entities controlling operation according to the MAC protocol atcentral access point 102 and subscriber units 104 are referred tocollectively as the MAC layer. This identifies these entities ascollectively representing a layer in a multi-layer communication model.In reference to the well-known OSI multi-layer model of datacommunications, the MAC layer as it is discussed here corresponds to alowvest sublayer of the data link layer. Underneath the MAC layer is thephysical layer which is responsible for transmitting and receiving bitsover the wireless channel. The MAC layer implements a TDMA scheme forupstream communication. Each of one or more frequencies is divided intoa series of frames or minislots in the time domain.

FIG. 2 depicts interactions between central access point 102 and one ofsubscriber units 104. Central access point 102 includes a central accesspoint MAC layer block 202 and a central access point physical layerblock 204. Subscriber unit 104 includes a subscriber unit physical layerblock 206 and a subscriber unit MAC layer block 208.

In one embodiment, central access point MAC layer block 202 andsubscriber unit MAC layer block 208 collectively operate according tothe MCNS protocol. In MCNS applications, central access point MAC layerblock 202 may be a BCM3210B integrated circuit available from Broadcom,Inc. of Irvine, Calif. Subscriber unit MAC layer processor 212 may be aBCM3300 integrated circuit provided by Broadcom.

Data and MAC layer network management information that are passedbetween MAC layer blocks 202 and 208 pass through physical layer blocks204 and 206 which are directly responsible for exchange of bits acrossthe wireless channel. Central access point 102 has exclusive access toat least one frequency for downstream transmissions. Subscriber unit104, however, shares access to one or more upstream transmissionfrequencies in accordance with the operative MAC protocol.

One class of network management messages exchanged between centralaccess point MAC layer block 202 and subscriber unit MAC layer block 208implements ranging, the process of establishing the round trippropagation delay between central access point 102 and subscriber unit104. Ranging requests including time stamps showing the current clockvalue at central access point 102 are sent downstream periodically tosubscriber unit 104. Ranging responses sent from subscriber unit 104 tocentral access point 102 assist central access point 102 in determiningthe round trip propagation delay between central access point 102 andsubscriber unit 104. Ranging responses occur at times reserved bycentral access point MAC layer block 202 for upstream rangingtransmission from subscriber unit 104.

In MCNS, central access point 102 measures the power level of the MAClayer ranging response messages received from subscriber unit 104.Although the power adjustment process that occurs concurrently withranging is controlled by central access point MAC layer block 202, thepower measurement may be made by central access point physical layerblock 204. In response to each such power measurement made on a rangingresponse message, central access point MAC layer block 202 formulatespower adjustment information to transmit to subscriber unit 104. Thepower adjustment information transmitted downstream may be, e.g., theraw power measurement, or a desired transmitted power level oradjustment. Subscriber unit 104 adjusts its power output in response tothe power adjustment information. Techniques for adjusting power aredescribed in the co-filed, co-assigned application entitled MULTI-LOOPPOWER CONTROL. The repetition of these power measurements and poweradjustments implements an automatic level control (ALC) loop thatregulates the output power of subscriber unit 104. Subscriber unit MAClayer block 208 receives the power adjustment information and directssubscriber unit physical layer block 206 to modify transmitter power inresponse.

Central access point MAC layer block 202 and subscriber unit MAC layerblock 208 also act as data interfaces to higher layers. Application dataincluding, e.g., voice, video, computer files, etc. is exchanged betweencentral access point MAC layer block 202 and subscriber unit MAC layerblock 208.

Downstream transmissions from central access point MAC layer block 202to subscriber unit MAC layer block 208 include a MAC layer address ofsubscriber unit 104 to identify the intended recipient. Upstreamtransmissions from subscriber unit MAC layer block 208 to central accesspoint MAC layer block 202 may be identified by a source address or bytheir position and time.

When subscriber unit MAC layer block 208 has application data ready forupstream transmission to central access point MAC layer block 202, ittransmits a special network management message known as an accessrequest (RA). MCNS allocates certain time domain slots for transmissionof access requests. Since an access request does not contain very muchinformation, the slot reserved for the access request may be very short.Access request slots are not allocated to particular subscriber units.Instead, there is a possibility that multiple subscriber units willsimultaneously issue access requests within the same time slot causingwhat is known as a collision.

A MAC protocol such as MCNS provides mechanisms for handling suchcollisions where each subscriber unit upon detecting an occurrence of acollision delays retransmission by an independently calculated pseudorandom time interval. In orthogonal frequency division multiplexing(OFDM) systems, multiple subscriber units may transmit access request inthe same time slot by employing different frequency subchannels of thecommon upstream frequency allocation. Such a scheme is disclosed in thepatent application, U.S. app. Ser. No. 09/019,938 and in the co-filedpatent application entitled EFFICIENT REQUEST ACCESS FOR OFDM SYSTEMS.

In response to an access request, central access point MAC layer block202 sends a grant downstream to subscriber unit MAC layer block 208. Thegrant notifies subscriber unit MAC layer block 208 of the upstream timeslot or slots reserved for transmission by subscriber unit 104.

According to the present invention, ALC of subscriber unit transmissionpower may be enhanced using a power adjustment sequence that occurs inresponse to MAC layer access requests. Power adjustment commands may bebased on power measurements made on either the access requeststhemselves, special upstream power measurements that are scheduled inresponse to access requests, or some combination of the two. Poweradjustment commands generated in this way help implement an ALC loopthat has a faster response time than the ALC loop that is based on powermeasurements of ranging responses. Such fast loop power adjustmentcommands more accurately reflect current channel conditions at the timeof an upstream data transmission. They provide much greater tolerance tofading channels than would be provided by reliance on the MAC layerpower adjustment commands based on measurements of the rangingresponses. Central access point physical layer block 204 and subscriberunit physical layer block 206 include systems for encoding, decoding,demodulating and modulating to implement radio communication betweencentral access point 102 and subscriber unit 104. Subscriber unitphysical layer block 206 is also capable of controlling its transmitteroutput power level.

A central access point physical layer control processor 214 isresponsible for formulating power adjustment commands in response topower measurements made on power measurement transmissions, data, andaccess requests. A subscriber unit physical layer processor 216 isresponsible for formulating power measurement transmissions and foradjusting power in response to power adjustment commands andinstructions received from subscriber unit MAC layer block 208 andphysical layer control processor 214.

FIG. 3 depicts the location of upstream power measurements and time fora particular subscriber unit. The horizontal axis corresponds to time.The vertical axis corresponds to a magnitude of channel response. Eachsquare marked “Ranging” indicates a power measurement made on anupstream ranging response. A square marked “RA” denotes a powermeasurement made on an upstream access request. A square marked “Data”indicates a time when central access point 102 receives a datatransmission from subscriber unit 104.

Subscriber unit 104 may regulate the transmission power of the accessrequest based on previous MAC layer power adjustment commands receivedfrom central access point 102. These commands were in turn generatedbased on measurements of previous upstream ranging responses. The powerof the upstream data transmission is, however, set based on a poweradjustment command which was generated based on a measurement of theaccess request power. It can be seen that the access request is muchcloser in time to the data transmission than the most recent rangingresponse. Therefore, the data transmission power will more closelyreflect current channel conditions than it would if it were based on aMAC layer power adjustment command.

FIG. 4 is a flowchart describing steps of controlling subscriber unitoutput power according to one embodiment of the present invention. Atstep 402, subscriber unit 104 sends an access request to central accesspoint 102. At step 404, central access point 102 decodes the accessrequest. If there is a collision, or if excessive noise and/orinterference corrupts the access request, the access request may not besuccessfully decoded. In the system described here, the likelihood of anaccess request not being successfully decoded is higher than thislikelihood for a data transmission. This is because the power level forthe access request will be based on a previous ranging response powermeasurement or will be uncontrolled. If channel conditions havedeteriorated significantly since the last ranging response or if thenoise level is too high for successful transmission of the uncontrolledpower level, the access request will not be successfully transmitted. Ifthis occurs, the access request may be repeated. It should be noted thatadditional redundancy may be employed in transmitting access requestdata because the amount of access request data is relatively small. Thiswill make the access request robust to impaired channel conditions andreduce the need for repeat transmissions.

At step 406, central access point 102 measures the power in the accessrequest. This power measurement is performed within central access pointphysical layer block 204. Physical layer control processor 214formulates power adjustment information for subscriber unit 104 to helpbring its power to the desired level. At step 608, physical layercontrol processor 214 sends the power adjustment information tosubscriber unit 104. The command is received by subscriber unit 104 andphysical layer control processor 216 responds by performing poweradjustment computations and adjusting the subscriber unit transmitterpower level. Details of power adjustment computations are disclosed inthe patent application entitled POWER REGULATION USING MULTI-LOOPCONTROL. Details of communicating physical layer control informationdownstream are disclosed in the patent application entitledCOMMUNICATION OF PHYSICAL LAYER CONTROL PARAMETERS.

At step 410, central access point 102 sends a MAC layer grant tosubscriber unit 104 scheduling a time for the subscriber unit's datatransmission. The grant may schedule multiple slots for the transmissionof data or just one. At step 412, subscriber unit 104 transmits data tocentral access point 102 during the first assigned slot. The datatransmission occurs at the newly adjusted power level. At step 414,central access point 102 measures the power of this data transmissionand formulates new power adjustment information. The new poweradjustment information is intended to control the output power used forsubsequent data transmissions scheduled in the grant of step 410. Atstep 416, central access point 102 sends the further power adjustmentinformation to subscriber unit 104. A new desired power adjustment iscomputed by subscriber unit 104 based on this power adjustmentinformation.

Steps 412, 414, and 416 may repeat for as many data transmission slotsare reserved by the grant of step 410. Steps 414 and 416 need not beperformed for the last data slot transmitted upstream and they need notbe performed at all if the grant of step 410 only reserves one data slotfor transmission.

In an alternative embodiment, subscriber unit 104 sends a special powermeasurement transmission to central access point 102 after transmittingan access request but before transmitting data. Adjustment of thesubscriber units output power level for the data transmission depends inpart then on a power measurement made by central access point 102 on thespecial power measurement transmission. The power measurementtransmission will be closer in time to the data transmission than theaccess request. Therefore, the output power level for the datatransmission is set even more accurately than if the power adjustmentinformation transmitted to subscriber unit 104 is based on the accessrequest transmission.

The power measurement transmission may be in a very short time slotreserved for use by the subscriber unit. Alternatively, in an ODFMsystem, multiple subscriber units may transmit within the same powermeasurement slot by using independent frequency domain subchannels. Sucha power measurement transmission scheme is disclosed in the patentapplication entitled REALTIME POWER CONTROL IN OFDM SYSTEMS.

FIG. 5 illustrates the position of power measurements in the time domainin this alternative embodiment. As in FIG. 3, the horizontal axisrepresents time while the vertical axis represents a magnitude ofchannel response. Labeled squares indicate the times of upstream rangingresponses, an access request, and a data transmission. Additionally,there is a power measurement transmission between the access request andthe data transmission.

FIG. 6 is a flow chart describing steps of regulating subscriber unitoutput power in this alternative embodiment. Steps 602 through 608 areessentially the same as steps 402 through 408. At step 610, centralaccess point 102 sends a MAC layer grant to subscriber unit 104. Thegrant includes not only information identifying reserved time slots forone or more data transmissions but also information identifying timeslots for one or more special power measurement transmissions. At step612, subscriber unit 104 sends a power measurement transmission tocentral access point 102. At step 614, central access point 102 measuresthe power of this power measurement transmission. At step 616, centralaccess point 102 formulates and sends power adjustment information tosubscriber unit 104. Subscriber unit responds by adjusting poweraccordingly.

At step 618, subscriber unit 104 transmits data at the adjusted powerlevel. At steps 620 central access point 102 measures power of the datatransmission. A new power adjustment is then estimated based on themeasured power of the data transmission. Alternatively, there may befurther special power measurements preceding the next data transmission.The power adjustment would then be based on either one or more powermeasurement transmissions or the combination of the measured power levelof the data transmission and the measured power level of the one or morepower measurement transmissions. At step 622, central access point 102sends power adjustment information to subscriber unit 104 and subscriberunit 104 responds by adjusting its power for the next data transmission.

Step 618, 620 and 622 may repeat for as many data slots as are reservedby the grant of step 610. Preferably, power measurement transmissionsare also reserved within the grant of the step 610. Steps 620 and 622need not be repeated after the last data transmission and need not beperformed at all if there is only one data slot reserved in the grant ofstep 610.

FIGS. 7A-7B depict alternative structures for an access requesttransmission in an OFDM system where available spectrum is divided intoorthogonal frequency domain subchannels. FIG. 7A depicts a frequencydomain structure 702 of an access request transmission. The accessrequest is a single burst in an OFDM communication system occupying onetime domain slot allocated by the MAC layer. The structure of the accessrequest burst is along the lines described in U.S. app. Ser. No.09/019,938. In FIGS. 7A-7B, four different subscriber units share theaccess request burst. Each of the four subscriber units employs adifferent nonoverlapping subgroup of tones so that all four subscriberunits may request access simultaneously without causing a collision. Inone embodiment, each subscriber unit randomly selects which subgroup touse for each new access request. Alternatively, upon initialregistration to central access point 102, each of subscriber units 104is assigned one of the four tone subgroups for use in requesting access.

FIG. 7A depicts representative frequency domain structure 702 of anaccess request transmission implemented as a burst of 32 symbols. Thehorizontal axis of FIG. 7A represents frequency and each labeledrectangle represents one of the 32 symbols of the burst. Each subscriberunit transmits both training symbols and data symbols. Each subscriberunit's symbols are distributed over the spectrum. In this configuration,each subscriber unit should transmit a sufficient number of trainingsymbols having predetermined values for central access point 102 tocharacterize both the magnitude and phase of the channel responsebetween central access point 102 and the particular subscriber unit overthe entire channel. This channel response characterization is not forpower control purposes but rather to support receiver signal processingto assure that access request information is received correctly. Thenumber of training symbols for each subscriber unit should be at least vwhere v is the maximum expected duration of the channel impulse responsefrom any subscriber unit to central access point 102. The technique fordetermining the channel response based on v training symbols isdisclosed in U.S. patent app. Ser. No. 09/234,929, the contents of whichare herein incorporated by reference.

In the representative embodiment shown in FIG. 7A, each subscriber unitis allocated eight symbols of the burst, four for data and four fortraining. Symbols reserved for training are denoted by “Tx” where xidentifies a particular one of the four subscriber units sharing theburst (1,2 3, or 4). “Dx” denotes symbols reserved for data. Centralaccess point 102 measures the power level for each subscriber unitseparately using the received levels of the training symbols. Centralaccess point 102 recovers each data symbol by dividing the received datasymbol value by the channel response value for the data symbol'sposition in the band.

FIG. 7B depicts an alternative frequency domain structure 704 for anaccess request burst. The access request burst of FIG. 7B also includes32 symbols. In frequency domain structure, 704, however, each subscriberunit's data and training symbols are contiguous within a reservedsub-band. Instead of using the channel estimation technique disclosed inU.S. app. No. 09/234,929, central access point 102 instead divides thereceived value of each training symbol by the known transmitted value toobtain the channel response magnitude and phase for each training symbolposition. Channel response values for the remaining symbols aredetermined by interpolation. Since each subscriber unit transmitsprimarily in a limited sub-band, for channel training purposes itsuffices that each subscriber unit transmits a sufficient number oftraining symbols to characterize the channel response within its ownsub-band only. In the representative embodiment of FIG. 7B, only twotraining symbols are required for each subscriber unit. Details of thechannel estimation technique to be employed with the access requestburst of FIG. 7B are described in the patent application entitledEFFICIENT REQUEST ACCESS FOR OFDM SYSTEMS.

However, to optimally control subscriber unit output power, it ispreferable that power measurement for each subscriber unit be based onsymbols spread throughout the band. Accordingly, in addition totransmitting data and training symbols within a reserved sub-band, eachsubscriber unit also transmits supplemental power measurement symbolsoutside its own sub-band. The power measurement symbols are not used fortraining and do not carry access request data. To determine the receivedpower level for a particular subscriber unit, central access point 102uses the magnitudes of the training symbols and power measurementsymbols. The data symbols may be used for measuring power if they haveconstant magnitude as in e.g., a QPSK scheme. It should be noted thatthe magnitude values used for power measurement are preferably notprocessed to correct for the measured channel response.

In FIG. 7B, symbols reserved for data and training are marked as in FIG.7A while the supplemental power measurement symbols are marked as “Px”where x identifies the subscriber unit. Because of the reduced trainingoverhead, each subscriber unit may now include 5 data symbols in itsaccess request rather than 4 as in FIG. 7A, permitting transmission ofmore information relating to the access request. According to thepresent invention, one may also employ the lower training overhead toallow more subscriber units to share the access request burst.

It will be understood that the examples and embodiments described hereinare for illustrative purposes only and various modifications and changesin like thereof will be suggested to persons skilled in the art and areto included within the spirit and purview of this application and thescope of the appended claims. For example, the present invention may beapplied to wireline systems. All publications, patents, and patentapplications cited herein are hereby incorporated by reference.

What is claimed is:
 1. In a point to multipoint communication system, amethod for operating a central access point, said method comprising:receiving an access request transmission from a subscriber unit;performing a power control sequence to control output power of saidsubscriber unit in response to said access request transmission, whereinsaid power control sequence comprises: measuring power of said accessrequest transmission; formulating power adjustment information based onmeasured power of said access request transmission; and in response tomeasured power of said access request transmission, sending said poweradjustment information to said subscriber unit to control power of afuture transmission from said subscriber unit; and wherein said accessrequest transmission is included within an OFDM burst that includesaccess request information from said subscriber unit and anothersubscriber unit.
 2. The method of claim 1 further comprising: inresponse to said access request transmission, sending a grant messagescheduling a time slot for said future transmission.
 3. The method ofclaim 2 wherein said future transmission comprises a data transmission.4. The method of claim 2 wherein said future transmission comprises apower measurement transmission that is part of said power controlsequence and said grant message further schedules a time slot of a datatransmission, said power control sequence further comprising: measuringpower of said power measurement transmission; and in response tomeasured power of said access request transmission, sending a furtherpower adjustment information to said subscriber unit to control power ofa next transmission from said subscriber unit.
 5. The method of claim 4wherein said future transmission is said data transmission.
 6. Themethod of claim 1 further comprising: receiving network managementtransmissions from said subscriber unit at substantially fixedintervals; measuring power of said network management transmissions assaid network management transmissions are received; and in response tomeasured power of said network management transmissions, sending poweradjustment information to said subscriber unit at intervalssubstantially similar to said fixed intervals.
 7. The method of claim 1wherein said access request information from said subscriber unit isincluded only within a contiguous subband of said OFDM burst.
 8. Themethod of claim 7 further comprising: measuring power of said subscriberunit by measuring power of at least one symbol inside said contiguoussubband and at least one symbol outside said contiguous subband.
 9. In apoint to multipoint communication system, a method for operating asubscriber unit comprising: sending an access request transmission to acentral access point; receiving a first power adjustment informationindicator from said central access point, said first power adjustmentinformation indicator having been generated based on measured power ofsaid access request transmission; receiving a scheduling transmissionfrom said central access point, said scheduling transmission having beengenerated in response to said access request transmission, saidscheduling transmission specifying a time slot of a future transmissionby said subscriber unit; and sending said future transmission to saidcentral access point at a power level determined responsive to saidfirst power adjustment information indicator; and wherein said accessrequest transmission is included within an OFDM burst that includesaccess request information from said subscriber unit and anothersubscriber unit.
 10. The method of claim 9 wherein said futuretransmission comprises a data transmission.
 11. The method of claim 9wherein said future transmission comprises a power measurementtransmission.
 12. The method of claim 11 wherein said schedulingtransmission further specifies time of a data transmission.
 13. Themethod of claim 12 further comprising receiving a second poweradjustment information indicator from said central access point, saidsecond power adjustment information indicator having been generated inresponse to said power measurement transmission; and sending said datatransmission to said central access point at a power level determinedresponsive to said second power adjustment information indicator. 14.The method of claim 9 further comprising: sending network managementtransmissions to said central access point at substantially fixedintervals; receiving power adjustment information indicators based onsaid network management transmissions; from said central access point,said power adjustment information indicators being generated in responseto power measurements made on said network management transmissions; andadjusting power levels of said network management transmissions inresponse to said adjustment information indicators.
 15. The method ofclaim 9 wherein said access request information for said subscriber unitis included only within a contiguous subband of said OFDM burst.
 16. Themethod of claim 15 further comprising: transmitting at least one powermeasurement symbol outside of said contiguous subband.
 17. In a point tomultipoint communication system, a system for operating a central accesspoint, said system comprising: a MAC layer processor that receives anaccess request transmission from a subscriber unit; a physical layercontrol processor that formulates power adjustment information for saidsubscriber based on measured power of said access request transmission;and a physical layer block that sends said power adjustment informationto said subscriber unit to control power of a future transmission fromsaid subscriber unit; and wherein said access request transmission isincluded within an OFDM burst that includes access request informationfrom said subscriber unit and another subscriber unit.
 18. The system ofclaim 17 wherein said physical layer control processor, in response tosaid access request transmission, sends a grant message scheduling atime for said future transmission.
 19. The system of claim 18 whereinsaid future transmission comprises a data transmission.
 20. The systemof claim 18 wherein said future transmission comprises a powermeasurement transmission that is part of said power control sequence,wherein said grant message further schedules a data transmission, saidphysical layer block measuring power of said power measurementtransmission, and wherein said physical layer control processor, inresponse to measured power of said access request transmission, sendsfurther power adjustment information to said subscriber unit to controlpower of a next transmission from said subscriber unit.
 21. The systemof claim 20 wherein said future transmission is said data transmission.22. The system of claim 17 wherein said MAC layer processor receivesnetwork management transmissions from a subscriber unit at substantiallyfixed intervals, measures power of said network management transmissionsas said network management transmissions are received, and, in responseto measured power of said network management transmissions, sends poweradjustment information indicators to said subscriber unit at intervalssubstantially similar to said fixed intervals.
 23. The system of claim17 wherein said access request information from said subscriber unit isincluded only within a contiguous subband of said OFDM burst.
 24. In apoint to multipoint communication system, a system for operating asubscriber unit comprising: a MAC layer processor that sends an accessrequest transmission to a central access point; and a physical layercontrol processor that receives a first power adjustment informationindicator from said central access point, said first power adjustmentinformation indicator having been generated based on measured power ofsaid access request transmission; wherein said MAC layer processorreceives a scheduling transmission from said central access point, saidscheduling transmission having been generated in response to said accessrequest transmission, said scheduling transmission specifying a timeslot of a future transmission by said subscriber unit; wherein saidsubscriber unit sends said future transmission to said central accesspoint at a power level determined responsive to said first poweradjustment information indicator; and wherein said access requesttransmission is included within an OFDM burst that includes accessrequest information from said subscriber unit and another subscriberunit.
 25. The system of claim 24 wherein said future transmissioncomprises a data transmission.
 26. The system of claim 24 wherein saidfuture transmission comprises a power measurement transmission.
 27. Thesystem of claim 26 wherein said scheduling transmission furtherspecifies time of a data transmission.
 28. The system of claim 27wherein said physical layer control processor receives a second poweradjustment information indicator from said central access point, saidsecond power adjustment information indicator having been generated inresponse to said power measurement transmission, and wherein saidsubscriber unit sends said data transmission to said central accesspoint at a power level determined responsive to said second poweradjustment information indicator.
 29. The system of claim 24 whereinsaid MAC layer processor sends network management transmissions to saidcentral access point at substantially fixed intervals, receives poweradjustment information indicators from said central access point, saidpower adjustment information indicators being generated in response topower measurements made on said network management transmissions, andadjusts power levels of said network management transmissions inresponse to said slow control loop power adjustment informationindicators.
 30. The system of claim 24 wherein said access requestinformation from said subscriber unit is included only within acontiguous subband of said OFDM burst.